How to scale a light's size , and strength proportionally with the rest of the scene in Cycles?
You need to scale the light intensity according to the inverse square law. [Calculator] 


When dividing scene size by 6, divide light power by 36.
When dividing scene size by 10, divide light power by 100




Important note:
Point and Spot lights don't change their Size when scaled with the scene. You need to change their Size manually in the Lights panel. Area lights scale properly with the rest of the scene.



Example:
Scene scale divided by 6. Point light Size also divided by 6. Point light power divided by 36.